ATmega48/88/168
Note that enabling the alternate function of some of the port pins does not affect the use of the
other pins in the port as general digital I/O.
13.2 Ports as General Digital I/O
The ports are bi-directional I/O ports with optional internal pull-ups. Figure 13-2 shows a func-
tional description of one I/O-port pin, here generically called Pxn.
Figure 13-2. General Digital I/O(1)
PUD
Q
D
DDxn
Q CLR
WDx
RDx
RESET
1
0
Q
D
Pxn
PORTxn
Q CLR
RESET
WPx
WRx
SLEEP
RRx
SYNCHRONIZER
RPx
D
Q
D
L
Q
Q
PINxn
Q
clk I/O
WDx:
RDx:
WRx:
RRx:
RPx:
WPx:
WRITE DDRx
READ DDRx
WRITE PORTx
PUD:
PULLUP DISABLE
SLEEP CONTROL
I/O CLOCK
SLEEP:
clkI/O
:
READ PORTx REGISTER
READ PORTx PIN
WRITE PINx REGISTER
Note:
1. WRx, WPx, WDx, RRx, RPx, and RDx are common to all pins within the same port. clkI/O
,
SLEEP, and PUD are common to all ports.
13.2.1
Configuring the Pin
Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. As shown in “Register
Description” on page 88, the DDxn bits are accessed at the DDRx I/O address, the PORTxn bits
at the PORTx I/O address, and the PINxn bits at the PINx I/O address.
The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one,
Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input
pin.
If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is
activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to
be configured as an output pin. The port pins are tri-stated when reset condition becomes active,
even if no clocks are running.
73
2545M–AVR–09/07
ATMEL [ ATMEL ]
